1. Field of the Invention
The present invention relates to a circular polarized microstrip antenna having a dielectric substrate with a patch electrode formed on one surface thereof, and a ground electrode formed on another surface thereof.
2. Description of the Prior Art
Recently, there has been an active move to incorporate a GPS antenna in portable equipment to thereby build a portable navigation system or obtain position information and the like by Cellular Phone in urgent communications, resulting in an increasing demand for very small-sized antennas.
FIG. 11 is a plan view of a conventional circular microstrip antenna 101 in wide use. The microstrip antenna 101 has a nearly square dielectric substrate 104 with a nearly square patch electrode 102 formed on one surface thereof, and a ground electrode (not shown) formed on almost the whole of another surface thereof. The patch electrode 102 has a feeding point 105 formed slightly away from the center thereof, to which power is fed through a coaxial cable (not shown) from the ground electrode. The patch electrode 102 has a pair of notches 102a and 102b formed so that they are positioned 135 and 315 degrees, respectively, with respect to a direction toward the feeding point 105 from the center of the patch electrode 102 , which is defined as 0 degree. These notches 102 and 102b, called retraction-separation elements, function to separate two modes (M1 and M2 in FIG. 11) perpendicular to each other, retracted in the microstrip antenna 101, and enable the microstrip antenna 101 to send or receive right-handed circular polarized radio waves.
In the square microstrip antenna 101 thus configured, its resonance frequency fr is generally given by the following expression (1).
[Expression 1] ##EQU1##
In the expression (1), c is a light speed, .di-elect cons.r is the relative dielectric constant of a relative dielectric substrate 104, h is the thickness of the relative dielectric substrate 104, and a is the length of one side of the square patch electrode 102.
It will be appreciated from the above expression (1) that a small-sized microstrip antenna 101 is achieved by using the dielectric substrate 104 having a large relative dielectric constant .di-elect cons.r. For example, where the microstrip antenna 101 is used for GPS receiving, when .di-elect cons.r=20, the length of one side of the dielectric substrate 104 is approximately 25 mm, while, when .di-elect cons.r=90, the length of one side of the dielectric substrate 104 is reduced to approximately 12 mm. For this reason, as the dielectric substrate 104, microwave dielectric ceramics (hereinafter simply referred to as ceramics) having large relative dielectric constants .di-elect cons.r are often used.
FIG. 12 represents changes of resonance frequency fr for variations in the size of one side of a square patch electrode. In the drawing, the dashed line G is for the dielectric substrate when .di-elect cons.r=20, and the dashed line H is for the dielectric substrate when .di-elect cons.r=90. As seen from FIG. 12, the larger is the relative dielectric constant .di-elect cons.r, the greater are the changes of the resonance frequency fr for variations of the size of the patch electrode. Herein, size variations of the patch electrode affect not only the length of one side but also, e.g., the notches 102a and 102b, resulting in changing not only the resonance frequency fr but also a circular polarized wave generation frequency and even its axis ratio.
FIG. 13 represents changes of the resonance frequency fr for variations of relative dielectric constant .di-elect cons.r. In the drawing, the dashed line I is for the dielectric substrate when .di-elect cons.r=20, and the dashed line J is for the dielectric substrate when .di-elect cons.r=90. It will be appreciated from FIG. 13 that although the magnitude of relative dielectric constants contributes less in comparison with the case of FIG. 12, the larger is the relative dielectric constant .di-elect cons.r, the greater are the changes of the resonance frequency fr.
Therefore, although the above-described conventional microstrip antenna 101 is advantageous in that it can be miniaturized by using the dielectric substrate 104 having a large relative dielectric constant .di-elect cons.r, it is disadvantageous in that since it is greatly affected by variations in production quality and other factors, it is afflicted by resonance frequencies fr remarkably far from desired values, a large axis ratio, and other problems, resulting in reduced yields.
As a conventional method for solving these problems, a circular polarized microstrip antenna 110 as shown in FIG. 14 is proposed. The microstrip antenna 110 has a nearly square (or circular) patch electrode 112 formed on one surface of a dielectric substrate 114 wherein projections 116a to 116d for axis ratio adjustment, and projections 117a to 117d and conductor cutout portions 118a and 118b for frequency adjustment are formed in predetermined positions of the patch electrode 112. The projections 116a to 116d for axis ratio adjustment, which are retraction-separation elements, are formed 45, 135, 225, and 315degrees, respectively, with respect to a direction toward the feeding point 115 from the center of the patch electrode 112, which is defined as 0 degree. The projections 116a and 116c are formed longer than the projections 116b and 116d. The projections 117a to 117d for frequency adjustment are formed 0, 90, 180, and 270 degrees, respectively, and the conductor cutout portions 118a to 118d for frequency adjustment are formed in the vicinity of the bases of the projections 117a to 117d.
In the microstrip antenna 110 configured in this way, the projections 116a to 116d for axis ratio adjustment are each cut by an equal amount to adjust an axis ratio so that it becomes equal to or smaller than a defined value. If a resonance frequency after the axis adjustment is below a target frequency, the projections 117a to 117d for frequency adjustment are each cut by an equal amount to gradually increase the resonance frequency so that it becomes equal to the target frequency. If the projections 117a to 117d for frequency adjustment have been excessively cut to such an extent that the resonance frequency exceeds the target frequency, the conductor cutout portions 118a to 118d for frequency adjustment are cut to gradually decrease the resonance frequency so that it becomes equal to the target frequency.
On the other hand, if a resonance frequency after the axis adjustment is already equal to or greater than the target frequency, the conductor cutout portions 118a to 118d for frequency adjustment are cut to gradually decrease the resonance frequency so that it becomes equal to the target frequency. If the resonance frequency has decreased below the target frequency as a result of this operation, the projections 117a to 117d for frequency adjustment are each cut by an equal amount to gradually increase the resonance frequency so that it becomes equal to the target frequency.
As described previously, in the conventional microstrip antenna 110 shown in FIG. 14, since the projections 116a to 116d for axis ratio adjustment, and the projections 117a to 117d and conductor cutout portions 118a to 118d for frequency adjustment are formed in predetermined positions of the patch electrode 112, the projections 116a to 116d for axis ratio adjustment are cut to adjust the axis ratio so that it becomes equal to or smaller than the defined value, and then the projections 117a to 117d and conductor cutout portions 118a to 118d for frequency adjustment are cut, whereby the resonance frequency can be adjusted to the target frequency. However, the conventional microstrip antenna 110 has a problem in the following point. That is, the projections 116a to 116d for axis ratio adjustment, and the projections 117a to 117d and conductor cutout portions 118a to 118d for frequency adjustment do not function independent of each other, and even if the axis ratio has been set below the defined value by cutting the projections 116a to 116d for axis ratio adjustment, the axis ratio may be deteriorated again by subsequent cutting of the projections 117a to 117d and conductor cutout portions 118a to 118d for frequency adjustment. There is also a problem in that, if the projections 116a to 116d for axis ratio adjustment have been excessively cut carelessly, the rotation direction of circular polarized waves is reversed.